• Radio and TV stations have to synchronise times to a very high degree of accuracy.
• Radar works by measuring the length of time a radio signal takes to bounce off an object and return.
• The Global Positioning System (GPS) works by measuring the signals sent by GPS satellites high above the Earth.
• Even a very basic home computer needs to be able to measure time to an amazing degree of accuracy. For example, if a processor is working at 200 MHz, it completes a single operation in 5 nanoseconds (or 5 x 10–9 s).
Note that the kilogram is the only SI unit with the prefix as part of its name. Multiple prefixes are generally not used.
Measuring instruments
Throughout your study of physics, you will make many measurements. To do this accurately, you will need to decide which instruments to use and how to read them.
You will already know how to use rulers, trundle wheels and opisometers. In your Leaving Certificate Physics course, you will use some new instruments for measuring length. These are the micrometer, vernier callipers and vernier scales.
Micrometer anvil
spindle
lock nut
sleeve
thimble
ratchet
lower scale frame
FIG. 1.1 A micrometer is used to measure diameter
A micrometer (Fig. 1.1) measures diameter accurate to 0.01 mm. Each full revolution of the spindle measures 0.5 mm.
u How to use a micrometer to measure diameter
1. Tighten the ratchet until it clicks. 2. Make sure to record any zero error at this point. You need to add or subtract this from the final measurement.
3. Read the scale on the sleeve. (Add 0.5 mm if a mark is showing on the lower scale.) 4. Read the thimble scale and add this to the main scale.
Zero error is the amount you need to add or subtract to make your starting point 0. Due to wear and tear of equipment, some instruments need to be ‘zeroed’ before use.
4 FUSION
For example, the reading in Fig. 1.2 is: 11.00 0.50 0.17
11.67 mm (final measurement) FIG. 1.2 Measuring using a micrometer
(from the sleeve’s top scale) (from the sleeve’s lower scale) (from the thimble)
